Rapid colorimetric pyrazinamide susceptibility testing of Mycobacterium tuberculosis

SETTING: Colorimetric methods for detecting drug resistance in Mycobacterium tuberculosis are very attractive, as they are cheap, easy to use and require no costly apparatus. Although the current colorimetric methods have been used for other drugs, such as rifampicin and isoniazid, few of the colorimetric methods have been reported to have been used in performing drug susceptibility testing (DST) against pyrazinamide (PZA). OBJECTIVE: To develop a rapid and reliable colorimetric method for PZA DST using a monotetrazolium redox dye, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), as indicator of viability. DESIGN: A total of 50 clinical isolates and three standard strains were tested using this colorimetric method and the BACTEC (TM) MGIT (TM) 960 system. RESULTS: Initial test results showed that the PZA DST results were available in 4-6 days; the overall sensitivity and specificity of the CTC colorimetric method were respectively 97.1% and 81.3% in comparison with the MGIT 960 results. CONCLUSION: These results suggest that the CTC colorimetric method is most rapid among the current PZA DST methods based on culture, and could be used for determining susceptibility to PZA of M. tuberculosis isolates.